Why have separate sexes? is a fundamental question in biology and has been investigated intensively since Darwin first proposed two hypotheses: Separate sexes evolve 1) to avoid detrimental effects of self-fertilization or 2) to improve allocation of scarce resources to each sexs reproductive function. In animal-pollinated plants dioecy is hypothesized to be favored when small, generalized pollinators predominate because they increase self-fertilization rates of plants. I concurrently tested Darwins hypotheses using Echinocereus coccineus (Cactaceae). I found, as predicted, when specialized pollinators (hummingbirds) were excluded from plants and only generalized pollinators (bees) visited, selfing rates of plants increased 238%. High selfing rates produced by bees, along with high levels of inbreeding depression in E. coccineus populations, were sufficient to select for dioecy, even in absence of any reallocation of resources by the plants.
Hummingbirds may also maintain high levels of gene flow among E. coccineus populations: populations blooming synchronously with the hummingbird migration had lower FST values (i.e. less genetic divergence) than asynchronously blooming populations. Population genetic data also suggest frequency of dioecious populations should be negatively correlated with both hummingbird abundance and elevation.
Structural equation models of effects of hummingbird abundance, elevation and annual rainfall (resources) on dioecy across the range of E. coccineus supported this hypothesis. All predictor variables were either directly or indirectly negatively correlated with dioecy. Only models with hummingbird abundance as a direct predictor of dioecy significantly fit the data. Darwins first hypothesis was again supported: as hummingbird abundance declines (and selfing rates increase), the incidence of dioecious populations increases.
Finally, to determine how changes in pollinator type may be globally influencing the evolution of dioecy, I conducted a literature survey of studies noting changes in pollinator types along elevation gradients and compared these to studies of dioecy along elevation gradients. The abundance of specialized insect pollinators declines with elevation, often leaving only generalized pollinators at the highest elevations: Proportion of dioecious species in communities increases accordingly with elevation. I propose elevation may be useful as a worldwide predictor of dioecy because it effects pollinator community composition and amount of inbreeding depression expressed in plant populations.